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so, which one is correct? why?
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"I haven't met..." is correct. "Haven't" is past tense, so the corresponding verb must also be past tense, in this case "met".

"I haven't met you before."
"I haven't been to London."
"I haven't climbed Mount Everest."
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is HASN'T also a past tense?
Oops... sorry. I have to correct myself. "Have" is not past tense. However, when we are saying "have not" it can relate to a past tense situation. It is a present tense, referring to a present condition of something not experienced in in the past, for example:

"I have not eaten today."

The same goes for using "has" (has not / hasn't), which refers to third person (he/she/it):

"He has not eaten today."

See this Wiktionary Simple English link for the word "have": http://simple.wiktionary.org/wiki/have
Yayya Lautner so, which one is correct? why?
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